I will let others respond as to best source and spring rates, but I will say I bought mine as "HD" springs for a 400 convertible from JR Spring, and that purchase was made in 2001. I replaced my fronts at the same time. I don't know if they are still available, or if that company exists, and I don't know the rates of the springs. I also could not do a before-after comparo because one of my spring leafs was actually cracked in half and was therefore my bird was sagging lower on one rear side than the other so I had no baseline. I will say my replacements are very VERY STIFF, and they set the car so the rear is just a HAIR higher than the front. And that's even with my battery relocated to trunk.

I will address your other questions with my recent experience of actually installing them about 6-ish months ago:

1. Buy all new bushings (front and rear, although many new springs will already come with brand new rubber front bushings installed...so ask the spring supplier) I used Polyurethane, which are likely contributing greatly to my car's stiffness. But I'm building a bit of a rally racer type of setup here to I'm willing to trade no-slop performance for comfort. Many won't accept that trade for a driver and I believe most people would much prefer normal rubber bushings.

2. Buy 6 front spring pocket cage nuts. Yours will break upon removal, or will need to be torched off/destroyed.

3. Buy spring pads. When you see the originals removed from the car, you will consider them to not even be reusable. You'll see. Again I used Poly. Normal rubber will probably be better for most.

4. MAYBE buy: U-bolt and T-bolt kit. Totally depends on extent of rust and condition of yours. You're in CA...so if your car is relatively rust-free, no reason these can't be re-used. Also not terribly expensive and this is an ideal time to replace them. If you see anything bent or remotely questionable, just buy the kit.

5. DO NOT BUY: Repro shackles...unless your originals are physically damaged in some way. The original shackles are SOOOO much thicker/beefier/stronger/better than the thin-gauge repro shackles being sold by all the parts houses. When I saw them side-by-side I laughed out loud and tossed the repro shackles in my scrap metal pile. Take off your originals, blast/clean/repaint to bring them to your standards of appearance, and re-use, but you might need to buy new nuts & bolts depending on condition (I did). Cosmetically restore your front spring buckets and shock mount plates at the same time. Even though my car came from Chicago rust country and needed full replacement rear frame rails and floors, my original shackles were/are solid as a ROCK, and many times better/heavier than the repros. They're bullet-proof.

6. Use the torque specs and install procedures in the service manual. Don't just go by "feel" when tightening nuts and bolts.

7. No better time to service U-joints than when the driveshaft is already out of the car...and maybe a transmission output seal? Hard axle brake lines and hose, if you fully drop out the rear end while doing this? At least inspect this stuff and make your own call while it's apart.

And I'll close with a scary possibility that I do not wish on you. If yours happens to be a rust-belt car like mine was:

As stated, I bought my springs in 2001 and installed them in 2012. Why? When I removed one of my original spring front buckets, the entire frame rail/torque box section of the unibody came out with it as basically a pile of rust dust...so I needed far more than 6 cage nuts! So my planned weekend spring replacement project in 2001 became an unplanned 11 year full-frame-rail and almost-full-floor replacement project, that I have very recently wrapped up about 90%, and am FINALLY driving the car again but still working on full completion. That's one of the reasons I have become more active here lately...11 years of a disassembled car has led me to to FORGET a few things on how to rebuild it, and I needed some bits of advice from folks here who are familiar with it to help me over a few humps. I'm past most of them now, but always learning.

So those with rusty cars, BEWARE rear spring replacement. Never know what you might uncover. But mine was indeed a basket case and I somewhat knew of this risk going in.